Dance Anatomy & Kinesiology

44 Dance Anatomy and Kinesiology

which muscles have broad attachments, the arrow

would be drawn to bisect the broad attachment. In

some cases with very broad or multiple proximal or

distal attachments, multiple arrows may need to be

used, and different portions of the muscle may have

different actions due to different lines of pull rela-

tive to the axes of movement. In such instances, the

usual way of reflecting this difference in action is to

divide the muscle into portions such as the upper

(clavicular) and lower (sternal) portions of the

pectoralis major (figure 2.9A). Although simplistic,

and not inclusive of all of the factors involved in an

accurate mathematical analysis of a muscle’s line of

pull, this is an effective tool for deducing the actions

of a given muscle in the tradition of Platzer (1978)

and Kreighbaum and Barthels (1996). Readers are

encouraged to approach learning muscle actions in

this manner rather than by rote memorization to

make the learning process easier, to provide a self-

check procedure, and to promote retention. If just

the location and line of pull of a muscle are learned,

the actions can be figured out logically, or conjec-

tured actions can be evaluated. To facilitate this

process, summary figures showing the approximate

line of pull of primary muscles of their respective

joints are provided in chapters 3 through 7.

When applying this line of pull analysis to a spe-

cific movement, rather than a specific muscle, the

double-headed arrow is replaced by an arrow with

one head. The direction of this arrow reflects the pull

of the muscle on the body segment that is moving in

the given movement.

It is also important to realize that the line of

pull of a muscle may change its relation to the axis

of rotation of a given joint in different ranges of

motion, causing the muscle to change its action. For

example, the upper (clavicular) portion of the pecto-

ralis major acts as an adductor of the shoulder when

the arm is low as seen in figure 2.9B; but when the

arm is brought above shoulder height (90° shoulder

abduction), the line of pull shifts to above the axis

of rotation of the shoulder and the clavicular por-

tion of the pectoralis major becomes an abductor

of the shoulder as seen in figure 2.9C. In dance, the

same change in the line of pull occurs when raising

the arms to the side from low to high fifth, and the

dancer can palpate contraction of the pectoralis

major in the upper ranges of the movement.

Muscles, Levers, and Rotary Motion.

When force is applied to a system that is not restricted

at some point, motion occurs along a straight line;

that is, linear motion occurs. However, as previously

described, in the human body, bones are restricted at

joints; therefore when a muscle contracts, its attach-

ments produce rotation of the associated bone or

body segments about its joint axis or axes. This form

of motion is called rotary or angular motion, and it

occurs in all types of synovial joints except gliding

joints. Learning about angular motion and levers is

important for appreciating how changing the body

configuration, such as having the knee bent versus

straight, profoundly influences the forces required

to execute a movement.

FIGURE 2.9 Change in line of pull and action of the

pectoralis major with different angles of shoulder abduc-

tion.